Compensatory and statistical-averaging effects jointly explain the decrease in species asynchrony over 60 years of succession

As succession proceeds, theory predicts increasingly asynchronous dynamics among species, thereby enhancing community temporal stability. However, recent studies often report decreased species asynchrony in older communities, and it remains unclear whether these successional changes in asynchrony reflect ecological processes or statistical mechanisms. Using a 60-year vegetation time series from the Buell–Small Succession Study, we quantified temporal changes in species asynchrony during old-field succession. We then decomposed species asynchrony into compensatory effects (CPE) and statistical-averaging effects (SAE) to assess the relative importance of these components. We further examined how CPE and SAE are related to species richness and whether these relationships shift through time. We show that the long-term decline in species asynchrony over succession was jointly explained by decreases in both CPE and SAE. SAE consistently accounted for a larger proportion of species asynchrony, but CPE declined more strongly over succession and therefore contributed more to the overall decline in species asynchrony. We further show that SAE, but not CPE, consistently increased with species richness at all stages, indicating that losses of diversity in later successional stages were associated with lower SAE. Consequently, both the direct decline in CPE and the indirect decline in SAE via decreased richness contributed to the observed decrease in species asynchrony over succession. Synthesis. Our results provide clear evidence that both the compensatory dynamics and statistical-averaging mechanisms contribute to long-term decreases in species asynchrony during succession, and that the stabilizing role of diversity is largely linked to statistical averaging. These insights refine classical views of successional dynamics and deepen our understanding of how biodiversity–stability relationships unfold over time.